BYZANTINE CHURCHES CHAP. in the dome arches on three sides—the eastern dome arch being prolonged to form the bema. The dome arches have arcades .communicating with the ambulatory on the norths south3 and west. The vaulting is executed either with barrel or with cross-groined vaults. These churches are evidently planned from a centre, not, like the domed basilicas, from a longitudinal axis. At the same time the absence of any cross I 2 345 678 9 10 U 12 13 14 15METERS FIG. 5.—THE CHURCH OF THE KOIMESIS, NICAEA (Wulf). arms differentiates them from, the domed cross churches. S. Andrew, which still retains its western arcade, dates from at least the sixth century, so that the type was in use during the great period of Byzantine architecture. Indeed, we should be inclined to regard S. Andrew as a square form of SS. Sergius and Bacchus, but without galleries. The type is a natural development from the octagonal domed church with its surrounding ambulatory.